1. Field
Example embodiments relate to a resistive random access memory (RRAM) having a solid solution layer and method of manufacturing the same. Other example embodiments relate to a RRAM having a transition metal solid solution layer between an electrode and a resistive layer to stabilize voltage and resistance variations of the RRAM.
2. Description of the Related Art
Conventionally, a semiconductor memory array structure includes memory cells, which are connected to one another. Each of the conventional memory cells of a dynamic random access memory (DRAM), which represents a semiconductor memory, includes a switch and a capacitor. DRAMs exhibit high integration and fast operating speed. If the power supply is removed from the DRAM, then all of the stored data is erased from the DRAM.
A flash memory represents a non-volatile memory capable of preserving stored data from being erased after the power supply is removed. Flash memories have non-volatile characteristics that are different than a volatile memory. Flash memories have a lower degree of integration and a slower operating speed than a DRAM.
Examples of a non-volatile memory, which have been studied, include a magnetic RAM (MRAM), a ferroelectric RAM (FRAM), a phase-change RAM (PRAM), etc.
MRAMs and FRAMs have been the focus of recent research. MRAMs store data according to the change in the polarization direction of a tunnel junction. FRAMs store data using polarization characteristics of a ferroelectric layer. MRAMs and FRAMs have higher integration, faster operating speeds and increased data retention characteristics. MRAMs and FRAMs may be driven at a lower power.
PRAMs store data according to a change in the resistance value caused by a phase change of a specific material. PRAMs include a resistor and a switch (transistor). If a method of manufacturing a conventional DRAM is used to manufacture the PRAM, then etching is difficult to perform. The additional time needed to etch may lower productivity and increase unit cost of PRAM products. As such, the competitive power of PRAMs may decrease.
A resistive RAM (RRAM) stores data according to a change in a variable resistance characteristic of a transition metal oxide (i.e., a resistance value varying according to voltage).
FIG. 1A is a diagram illustrating a cross-sectional view of a conventional RRAM using a conventional resistive transformation material having a conventional structure.
Referring to FIG. 1A, the conventional RRAM includes a lower electrode 11, a resistive layer 12 and an upper electrode 13, sequentially formed. The resistive layer 12 may be formed of a transition metal oxide. The lower and upper electrodes 11 and 13 may be formed of electrode materials used for a conventional semiconductor memory device.
FIG. 1B is a graph illustrating a relationship between a voltage V applied to the lower and upper electrodes 11 and 13 and a current I flowing in the resistive layer 12. The resistive layer 12 may be formed of a transition metal oxide as described above.
Referring to FIG. 1B, the voltage V was gradually increased from 0V in order to measure variations in the current flowing in the resistive layer 12. The current flowing in the resistive layer 12 gradually increases for an applied voltage between 0V and about 1V. The current flowing in the resistive layer 12 decreases in response to an increase in resistance at a voltage V of about 1V. The current flowing in the resistive layer 12 increases in response to a further increase of the voltage V. A higher resistance state of the resistive layer 12 refers to a reset state and a lower resistance state of the resistive layer 12 refers to a set state.
In FIG. 1B, the voltage applied when the set state changes into the reset state is referred to as a reset voltage RV. The reset voltage is within a range between about 0.6V and 1.2V. The highest value of a resistance SR in the set state may be 10 or more times greater than the lowest value of a resistance SR in the set state. A set voltage SV, which is applied when the reset state changes into the set state, may be within a range between 1.5V and 4.6V. As such, the set voltage SV may have a higher variation value.
As described above, the voltage and the resistance variations of the conventional RRAM are higher due to an unstable resistance level of the resistive layer 12. Reliability of the conventional RRAM decreases due to unstable set and reset voltages, etc.